with val syntax sugar for more complex types

[jiribenes]

Problem statement

There's no way to unpack more complex types the using with val _ = ... syntax sugar.

Motivation

When working with stream, I tried to do something like:

def main() = {
  with val (i, x) = [(1, 'a'), (2, 'b'), (3, 'c')].foreach;
  println(show(i) ++ ": " ++ show(x))
}

in order to unpack the tuple.
However, that's wrong since this sugar would imply that foreach takes two arguments
as it desugars to something like:

...foreach { (i, x) => ... }

But the correct diet, sugar-free version is the following:

def main() =
  [(1, 'a'), (2, 'b'), (3, 'c')].foreach { case (i, x) => 
    println(show(i) ++ ": " ++ show(x))
  }

Possible and impossible workarounds

I tried to remedy this by using Tuple2(i, x) explicitly, but that reports a parse error:

def main() = {
  with val Tuple2(i, x) = [(1, 'a'), (2, 'b'), (3, 'c')].foreach;
  //             ^ Expected = but got (

  println(show(i) ++ ": " ++ show(x))
}

Similarly if I try to use a custom type:

record Pos(i: Int, x: Char)
def main() = {
  with val Pos(i, x) = [Pos(1, 'a'), Pos(2, 'b'), Pos(3, 'c')].foreach;
  //          ^ Expected = but got (

  println(show(i) ++ ": " ++ show(x))
}

What does work is the following:

def main() = {
  with val tup = [(1, 'a'), (2, 'b'), (3, 'c')].foreach;
  val (i, x) = tup;
  println(show(i) ++ ": " ++ show(x))
}

[jiribenes]

I'm also not 100% sure that we want this feature, but it's very helpful in the zip example in the PR text of #705.

[jiribenes]

Here's the code responsible for parsing with val ...:

effekt/effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

Lines 198 to 204 in adf76cf

	def withStmt(): Stmt = `with` ~> peek.kind match {
	case `val` =>
	val params = (`val` ~> peek.kind match {
	case `(` => valueParamsOpt()
	case _ => List(valueParamOpt()) // TODO copy position
	})
	desugarWith(params, Nil, `=` ~> expr(), semi() ~> stmts())

where as here's the code responsible for parsing val (a, b) = ...:

effekt/effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

Lines 398 to 403 in adf76cf

	def matchLhs() =
	`val` ~> matchPattern() ~ manyWhile(`and` ~> matchGuard(), `and`) <~ `=` match {
	case AnyPattern(id) ~ Nil =>
	val binding = stmt()
	val valDef = ValDef(id, None, binding).withRangeOf(startMarker, binding)
	DefStmt(valDef, { semi(); stmts() })